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Available
spreading codes Bandwidth requirements
of the user

Interference level
Bandwidth /
Quality of Service
Distance of the
user to the center
of the cell
Service level

Speed of the user

Figure 3.20 Factors influencing the Quality of Service and the maximum bandwidth of a connection.

3.5.3 Adaptive Multirate (AMR) NB and WB Codecs for Voice Calls
For UMTS, it was decided to use the Adaptive Multirate (AMR) codec for voice encoding,
which was previously introduced in GSM. With AMR, the codec is no longer negotiated
only at the establishment of a voice call; the system can change the codec every
20 milliseconds. As the name Adaptive Multirate suggests, this functionality is quite
useful in adapting to a number of changes that can occur during the lifetime of a call.
If the reception quality deteriorates during a call, the network can decide to use a
voice codec with a lower bit rate. If the spreading factor of the connection is not changed,
more bits of the bearer can then be used to add additional redundancy. A lower bit rate
codec naturally lowers the quality of the voice transmission, which is however still bet-
ter than a reduction in voice quality owing to an increased error rate. If the reception
quality increases again during the connection, AMR returns to a higher bit rate codec
and decreases the number of redundancy bits again.
Another application of AMR is to increase the number of simultaneous calls in a cell
during cell congestion. In this case, a higher spreading factor is used for a connection,
which only allows lower bit rate AMR codes to be used. This somewhat reduces the
voice quality for the subscriber but increases the number of possible simultaneous
voice calls.
Table 3.3 gives an overview of the different AMR codecs that have been standardized
in 3GPP TS 26.071 [8]. While UMTS mobile devices have to support all bit rates,
network support is optional.
A further significant voice‐quality improvement in mobile networks has been
achieved by introducing the AMR‐Wideband (AMR‐WB) codec. Although previous
AMR codecs, now also referred to as AMR‐Narrowband (AMR‐NB) codecs, digitized
the voice signal up to an audible frequency of 3400 Hz, AMR‐WB codecs have an upper
limit of 7000 Hz. Therefore, much more of the audible spectrum is captured in the
digitization process, which results in a much‐improved sound quality at the receiver
side. This, however, comes at the expense of the resulting data stream not being
compatible anymore with the PCM G.711 codec used in fixed‐line networks. In mobile